Patent Document 1 below discloses a water pump unit attached or mounted to the crankcase of an internal combustion engine at an outer surface of a vertical wall of an oil pan, wherein the water pump unit is driven by a drive shaft thereof joined coaxially to an oil pump drive shaft provided within the oil pan.
The water pump unit disclosed in Patent Document 1 has a disadvantage in that the vertical wall of the oil pan has to be formed with a large opening through which a part of the housing of the water pump unit is to be passed and an area occupied for joining associated component parts is enlarged. A further disadvantage is that a part of the drive shaft of the water pump unit is disposed within the crankcase so that assembling operation for the water pump unit is complicated.